1. Field of the Invention
The present invention relates to a scaleable and high-yielding method for producing a copolymer, which can be used as a component of a radiation-sensitive resin composition. More particularly, the present invention relates to a method for producing a copolymer that is suitable for a chemically-amplified resist which is useful for microfabrication utilizing various types of radiation such as, deep ultraviolet rays from a KrF excimer laser or ArF excimer laser, charged particle rays such as electron beams; and X-rays such as synchrotron radiation.
In the field of microfabrication represented by fabrication of integrated circuit (IC) devices, lithographic technology enabling microfabrication with a line width of 0.20 xcexcm or less has been demanded in order to increase the degree of integration. However, microfabrication with a line width of sub-quarter micron is very difficult using near ultraviolet (UV) rays, such as i-line radiation, which are used in a conventional lithographic process.
To deal with this problem, the use of a shorter wavelength radiation has been studied in order to enable microfabrication with a line width of 0.20 xcexcm or less. Examples of such shorter wavelength radiation include, but are not limited to: deep ultraviolet rays from a bright-line spectrum of a mercury lamp and an excimer laser; X-rays; electron beams; and the like. In particular, a KrF excimer laser (at 248 nm) and ArF excimer laser (at 193 nm) have attracted attention.
Various types of UV-sensitive compositions that utilize chemical amplification effects arising from an acid-dissociable-group-containing resin have been proposed, in which the resin includes a functional group which increases alkali solubility through the action of an acid and a component which generates an acid upon irradiation (exposure) (hereinafter called xe2x80x98photoacid generatorxe2x80x99). These radiation-sensitive resin compositions have been used as chemically-amplified resists.
For example, Japanese Patent Application Laid-open No. 7-234511 proposes a method of introducing an aliphatic ring into a resin component of the resist to produce a chemically-amplified resist having good radiation transmissivity (in comparison with a conventional phenol-based resin) and improved dry etching resistance.
A chemically-amplified positive-tone resist composition having high transparency at 193 nm, exhibiting high resolution, and being capable of forming a resist pattern of good pattern shape, dry etching resistance, and adhesion to substrates is proposed in Japanese Patent Application Laid-open No. 2001-242627. Specifically, a chemically-amplified resist using a resin having a unit derived from an acrylate or methacrylate is taught, of which the ester portion has a crosslinked saturated polycyclic hydrocarbon group containing a lactone ring in the main chain.
As an acid-dissociable-group-containing resin, Japanese Patent Application Laid-open No. 2002-23371 proposes a resin in which the unsaturated bond of an unsaturated alicyclic compound dissociates to make up part of the polymer main chain, for example.
As described above, acid-dissociable-group-containing resins may be obtainedby copolymerizing various types of monomers. However, even if monomers of differing molecular structure are copolymerized in a specific ratio in order toobtain a desired molecule of a resin suitable for use with short wavelength radiation (for example, deep ultraviolet rays), the resulting resin may have a monomer content differing from the expected value. In particular, the content of a monomer which forms a polymer main chain by dissociation of the unsaturated bond of an unsaturated alicyclic compound may be decreased in the resulting copolymer. This makes molecular design of a resin having high transparency to radiation and excelling in basic properties as a resist difficult.
In the case of copolymerizing cyclic olefin monomers and acrylic monomers under free radical conditions, the copolymerization is typically carried out using excess acrylic monomer as compared to cyclic olefin monomer. See, for example, U.S. Pat. Nos. 2,985,611; 3,536,681; and 3,697,490. In such copolymerizations, a low amount of cyclic olefin copolymer is incorporated into the matrix of the final copolymer or resin. It is noted that the terms xe2x80x9ccopolymerxe2x80x9d and xe2x80x9cresinxe2x80x9d are used interchangeably throughout this application.
In other copolymerizations of cyclic olefin monomers and acrylic monomers, a transition metal catalyst, i.e., co-ordination catalyst, or Lewis acid promoter is typically required to carry out the copolymerization. Such copolymerizations are described, for example, in U.S. Pat. Nos. 3,723,399; 6,111,041; 6,136,499; 6,232,417; 6,235,849; 6,300,440; and 6,303,724 as well as U.S. Patent Application Publication US2002/0040115A1 and the article to Tian, et al., entitled xe2x80x9cNeutral Palladium Complexes as Catalysts for Olefin-Methyl Acrylate Copolymerization; A Cautionary Talexe2x80x9d, Macromolecules 2001, 34, 7656-7663. A noted problem with using resins madefrom such metal catalysts is that the metal component frequently contaminates the final resin. Thus, a metal removal step must be used if the resins are to be used in the microfabrication of IC devices.
U.S. Pat. Nos. 5,585,219 and 5,585,222 disclose that electron-withdrawing substituents on the cyclic olefin monomer can facilitate an anionic copolymerization of the cyclic olefin monomer with acrylate or methacrylate.
U.S. Pat. No. 6,303,265 discloses that cyclic olefin copolymers may be made with monomers that possess two electron-withdrawing groups appended to the polymerizable double bond such as maleic acid and its esters. Specifically, in the ""265 patent, the copolymerization of the cyclic olefin is carried out in the presence of an electron poor monomer which contains two electron-withdrawing groups. The use of monomers containing two electron-withdrawing groups in conjunction with a cyclic olefin falls within the well-known regime of an alternating copolymerization. Previosuly disclosed copolymerizations of cyclic olefins with monomers containing one electron-withdrawing group are very limited and are discussed herein below.
U.S. Patent Application Publication US2002/0004570A1 discloses palladium, nickel, and free radical techniques for making cyclic olefin homopolymers having one or more pendant cyclic electron-withdrawing groups. In this reference, a complex cyclic olefin which includes a cyclic electron-withdrawing group that depends from a cyclic olefin is disclosed. No free-radical methods for preparing cyclic olefin-acrylate copolymers are disclosed in this application.
Although various methods of preparing resins including a cyclic olefin monomer are known, there is still a need for providing a new and improved method for producing a copolymer wherein the cyclic olefin monomer is copolymerized with another monomer that contains less than two electron-withdrawing groups attached thereto. Moreover, a method is needed wherein the amount of cyclic olefin being incorporated into the final resin is greater than heretofore possible with prior art free radical polymerization processes.
The present invention provides a simple, scaleable, high-yielding method to prepare copolymers of an unsaturated alicyclic monomer and an electron poor monomer which comprises reacting at least one monomer A which is an unsaturated alicyclic monomer and forms a polymer main chain by dissociation of an unsaturated bond, and at least one unsaturated monomer B, which also forms a polymer chain by dissociation of an unsaturated bond, wherein less than two electron-withdrawing groups are directly appended to said unsaturation, and where said monomer B is other than the unsaturated alicyclic monomer and forms a polymer main chain, in the presence of a free radical initiator, wherein said reacting is carried out using a stoichiometric excess of monomer A as compared to monomer B. By carrying out the reacting step in an excess of monomer A as compared to monomer B, the resultant copolymer will have a greater molar concentration of monomer A than is obtainable using prior art methods. One advantage of the present invention, among others, is that the inventive method allows for producing resins that have a glass transition temperature (Tg) of from about 120xc2x0 C. to about 180xc2x0 C.
It has been found that the reaction rate between monomers A is considerably slower than the reaction rate between monomer A and monomer B, which in turn is considerably slower than the reaction rate between monomers B. Similarly, the reaction rate between monomer B and monomer A is considerably slower than the reaction rate between monomers B. For example, although monomers A scarcely form a polymer, monomer A forms a copolymer in the presence of monomer B.
In a preferred aspect of the present invention, a copolymer of A and B can be obtained by copolymerizing the monomers under conditions in which the reaction mixture contains at least twice as much monomer A as compared to monomer B on a molar basis. In one embodiment, and in terms of a molar fraction, the resultant copolymer contains from about 15 to about 35% monomer A and from about 65 to about 85% monomer B. More preferably, the resultant copolymer contains, in terms of molar fraction, from about 25 to about 35% monomer A and from about 65 to about 75% monomer B.
In accordance with the present invention, monomer A, and monomer B may be added continuously, intermittently, or in stepwise fashion to a reaction vessel. In one embodiment of the present invention, additional monomer B may be added to the reaction after the copolymer has begun to form. In that particular embodiment of the present invention, the adding step may be performed such that monomer B is continuously being added to monomer A.
It should be noted that in the method of the present invention, the reaction occurs in the absence of any significant concentration of a Lewis acid promoter, such as a metal-chloride compound, or co-ordination catalyst. Because no significant amount of a Lewis acid promoter or co-ordination catalyst is present during the free radical copolymerization reaction, the resultant copolymer formed in the present invention does not need to undergo a metal removal process prior to being used as a component in a radiation-sensitive resist composition.
The inventive method thus allows for feasible, high-yielding syntheses of a resin in the absence of a Lewis acid promoter or co-ordination catalyst. Moreover, the inventive method greatly simplifies the synthetic procedure and isolation of a desired copolymer.